Vehicles include an exhaust gas treatment system for treating a flow of exhaust gas from an engine. The exhaust gas treatment system may include one or more catalytic devices that oxidize nitrogen oxides with a catalyst, such as a diesel particular filter or a selective catalytic reduction unit. Typically, a nitrogen oxides sensor is disposed downstream of the catalytic device, to sense the level of nitrogen oxides present in the flow of exhaust gas downstream of the catalytic devices. Feedback from the nitrogen oxides sensor(s) is used to control the operation of the engine and/or the exhaust gas treatment system. Accordingly, it is important to ensure that the nitrogen oxides sensor(s) are operating properly, and providing an accurate measurement of the nitrogen oxides in the exhaust gas. On-board diagnostics are used to determine if the nitrogen oxide sensor(s) are operating properly.
An on-board diagnostic system may compare a level of nitrogen oxides in the exhaust gas that was sensed or measured by the nitrogen oxides sensor, to an estimated value of nitrogen oxides expected to be present in the exhaust gas for the specific, current operating conditions of the engine. If the sensed level of nitrogen oxides is approximately equal to the estimated value of nitrogen oxides, then the on-board diagnostic system may determine that the nitrogen oxides sensors are operating properly. However, if the sensed level of nitrogen oxides is not approximately equal to the estimated value of nitrogen oxides, then the on-board diagnostic system may determine that the nitrogen oxides sensors are not operating properly, and may signal a maintenance alert.